This invention relates to the field of integrated circuits and, in particular, buffers and converters for integrated circuits.
Interfacing between various kinds of circuits, such as TTL, ECL or CMOS typically requires a converter or buffer between the types of circuits. These buffers or converters usually permit one type of circuit, such as TTL, to be coupled to another type of circuit, such as CMOS. These converters or buffers are necessary because one type of circuit will frequently operate at different voltage levels, have different current and drive capabilities, and have other different properties than another type of circuit (e.g. the current driving capacity of one circuit may poorly match with another type of circuit which may require more current to drive it).
These buffers or converters are prone to noise, such as voltage spikes, occurring on the power supply lines. That noise tends to induce false transitions in the converter or buffer which transitions may change the state of logic (such as a 1 to a 0). Solutions to this problem in the prior art have usually attempted to reduce the power supply noise. Such solutions include the decoupling and impedence matching of external power supply rails and the use of dual Vss pins and often the reduction of package inductance. The invention does not focus on reducing power supply noise but rather attempts to isolate it from the input buffer.
FIG. 1 illustrates a conventional input buffer (or converter) which may be used to convert between TTL outputs and CMOS input. That is, the voltage in (Vin) is obtained from a TTL output and appears at 1 of FIG. 1. The voltage out from the buffer, appearing at 6, may now be appplied to CMOS circuits. This prior art configuration of an input buffer will be discussed in more detail below. The invention permits a simple and inexpensive solution to the problem of power supply noise for input buffers. The invention allows true TTL input levels to drive high speed CMOS integrated circuit devices in electronic systems environments.